Image sensor arrays typically comprise a linear array of photodiodes which raster scan an image-bearing document and convert to the microscopic image area viewed by each photodiode to image signal charges. Following an integration period, the image signals are amplified and transferred to a common output line or bus through successively actuating multiplexing transistors.
U.S. Pat. No. 5,148,268 discloses a typical arrangement of a full-color image sensor array. Separate linear arrays of photosensors are arranged in parallel on a single bar, with the photosensors in each linear array being provided with a filter thereon of one primary color. The bar is caused to move relative to an original image in a scan direction which is generally perpendicular to the direction of the arrays. As the sensor bar moves along the original image, each portion of the area of the original image is exposed to each of the linear arrays of photosensors in sequence. As each array of photosensors moves past a particular small area in the original image, signals according to the different primary color separations of that area are output by one of the photosensors in each array. In this way three separate sets of signals, each relating to one primary color, are produced by the linear arrays of photosensors.
An important parameter in the design of an image sensor array is the resolution of the array, which will of course affect the quality of image signal based on an original image. One type of resolution is dictated by the physical configuration of the individual photosensors along the array: the higher the number of individual photosensors within a given unit of length along the array, the higher the possible resolution of data that may be output by the array. This "fast scan" or x-direction resolution is of course fixed by the size and spacing of the photosensors in the array.
Another type of resolution associated with an array is the "slow-scan," or y-direction, resolution, which is the resolution of the image along the direction perpendicular to the direction of the array, which would be the direction of an original image moving relative to the array. In contrast to the x-dimension resolution, which is fixed by the physical characteristics of the array, the y-direction resolution is determined by the speed of an original image relative to the array, coupled with the integration times of individual photosensors. If the original image is moving relative to the array at a constant velocity, and the photosensor is operating at a high speed, each integration time of the photosensor will cause exposure to a relatively small area on the original image; if the integration time is longer, with each integration time an individual photosensor will be "looking at" a relatively larger area of the original image. In brief, the shorter the integration time of an individual photosensor in the array, the higher the y-direction resolution of the array.
As will be described in detail below, a technical complication may result where the desired y-direction resolution, which is related to the integration times in an array, is different from the inherent x-direction resolution of the array. For example, one possible design for a full-page-width full-color array provides, by virtue of its photosensor size, a fixed 400 SPI resolution in the x-dimension, but can provide, by virtue of the operational speed of the photosensors, a 600 SPI resolution in the y-direction. The present invention is directed to physical and operating parameters of a full-color scanning array which overcomes certain design requirements caused by high y-direction resolution.